Construction machines such as hydraulic excavators include as a power source an engine that uses gasoline and light oil for its fuel, for example. This engine drives a hydraulic pump to generate hydraulic pressure and drives actuators such as hydraulic motors and hydraulic cylinders. Hydraulic actuators are small in size and weight but can output significant power, and for this reason, they are widely used as actuators for construction machines.
Construction machines such as hydraulic excavators include a swing structure. In a hydraulic excavator that uses a hydraulic motor to drive a swing structure, when a swing control lever returns to a neutral position during swing operation, a hydraulic line adapted to supply hydraulic fluid to the hydraulic motor is closed by a control valve. The swing structure is brought into a decelerated state by relief operation of a relief valve and then into a stop state.
In the conventional hydraulic excavators, all the energy of the hydraulic fluid discharged from relief valves was wasted as heat. Patent document 1 proposes an energy regeneration system in which a regeneration device composed of a hydraulic pump and an electric motor recovers the energy of the hydraulic fluid discharged from the relief valve and effectively uses it.
Patent document 1 has a safety valve installed between a swing hydraulic motor and the regeneration device. Only when an operation device is in a neutral state and a brake pressure not lower than a predetermined pressure is detected, the passage resistance in the safety valve can be reduced by an electric signal from a controller.